The long range aim of this program is the development of cell culture systems for carcinogenesis studies. Clonal growth of human and rodent fibroblasts in tissue culture is enhanced by lowering the dissolved oxygen concentration of the growth medium to 40-60 mm Hg. The enhanced growth is density-dependent and is associated with an increased proliferation rate rather than more efficient attachment and/or survival of the inoculated cells. In contrast, growth of human skin epithelium and rhes s monkey kidney cells is retarded by use of the lowered oxygen concentration. Similarly hemicypts ("domes") do not form in confluent monkey kidney cell cultures at the lower oxygen concentration. Means of limiting or prevented photodynamic oxidative cell chromosome damage are under study. Technical problems associated with the primary human epithelial cells for chemical and viral carcinogenesis and for mutagenesis studies are discussed.